Fair Value Measurements - Changes in Unrealized Gains (Losses) for Financial Instruments Classified as Level 3 Still Held (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	$ (16)	$ 3	$ (18)	$ 10
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	(69)	17	(130)	51
Changes in Unrealized Gains (Losses) Included in Income	79	(30)	78	(64)
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	$ 0	$ 0	$ 0	$ 0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Investments	Investments	Investments	Investments
Fortitude Re funds withheld payable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	$ 79	$ (30)	$ 78	$ (64)
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Bonds available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	0	0	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	(69)	17	(130)	51
Bonds available for sale \| Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	0	0	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	8	(13)	2	(13)
Bonds available for sale \| RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	0	0	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	(44)	15	(83)	30
Bonds available for sale \| CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	0	0	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	0	5	1	5
Bonds available for sale \| CLO/ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	0	0	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	(33)	10	(50)	29
Other bond securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	(2)	4	(3)	10
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Other bond securities \| RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	(1)	0	(1)	1
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Other bond securities \| CLO/ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	(1)	4	(2)	9
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	1	0	(2)	2
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in Unrealized Gains (Losses) Included in Income	(15)	(1)	(13)	(2)
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss)	$ 0	$ 0	$ 0	$ 0

Amount of unrealized gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

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Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

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Amount of unrealized gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3) and still held.

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